Keep it simple, stupid! A straightforward boot ROM for Freebee

I've been inflicting my buggy boot code with monitor etc on people, which is making their experiences in building Freebee more difficult. It's exacerbated by not distributing basic with the code, as that's copyright Microbee. I had one person wondering why it didn't do anything when he powered it up. After some faffing around, I asked "so did you patch basic in?" and it turned out he hadn't done that.

So I've generated a much simpler ROM that just copies the fonts in, then checks for the presence of basic (just looks for a C3h at 8000h). If it doesn't find that, it displays a helpful message as follows:

I shall put the new code, with a binary (not including basic!) in with the other design files.

MCLZ8 on Freebee

So it turns out you can emulate a Z80 using a Teensy 4.1. Better yet, with the aid of a few voltage translators on a little interposer board, you can plug the teensy into your Z80 machine and run using the teensy as the processor.

This is kinda huge - the sky's the limit for the debug ability that comes from being able to interrogate everything that's happening at processor level over a dead-basic USB connection.

Luckily, Freebee doesn't need no stinkin' coreboard to run as a basic 32K bee, so the rather tall Teensy + interposer board is no impediment. However the interposer board as layed out on the Microcore Labs website was a bit bulky, so I redid it so it fits neatly under the Teensy. This means a third board is also needed to get back to the normal 0.6" pin pitch for the Z80. I guess I could have used an SMD connector for both Teensy and Z80 socket, but it's easier with through-hole here.

Then we download the MCLZ8 firmware, install the Arduino IDE + Teensy loader, compile the source, and upload the code to the Teensy. And just like that, we have a Z80 hardware emulator in software.